Many prior art devices requiring power taps utilize couplers to tap power off of the input and/or output signal power of the device. More recently, other arrangements have been devised to provide optical power taps.
U.S. Pat. No. 5,748,815 (Hamel et al.), issued on May 5, 1998, discloses an optical component adapted to monitor a multi-wavelength link for use as an add-drop multiplexer in optical networks. The optical component includes an input optical fiber for launching a multiplexed signal comprising a plurality of different wavelength signals from a zero diffraction order of a flat blazed grating towards the grating for dispersing the wavelength signals in several non-zero orders of diffraction. A first set of optical fibers is located within a first non-zero diffraction order area (e.g., the 1 order) of the grating where each optical fiber of the first set is located to receive a separate one of the dispersed and demultiplexed wavelength signals. A second set of optical fibers is located within a second higher non-zero diffraction order area (e.g., the 2 order) of the grating. Each optical fiber in this set is located to receive a separate one of the dispersed and demultiplexed wavelength signals and transmit the wavelength signal to a separate one of a plurality of photodetectors for ascertaining the average power level coming out of each of these optical fibers. The Hamel et al. optical component is directed only for receiving a multiplexed wavelength input signal and obtaining therefrom demultiplexed wavelength signals within one diffraction area of a grating while concurrently detecting the power of the received demultiplexed wavelength signals within a second diffraction area of the grating. Wavelength drift can also be determined from the demultiplexed wavelength signals. The optical component does not provide for determining overall multiplexed signal power.
U.S. Patent Application Publication No. US 2002/0057875 A1 (Kaneko), published on May 16, 2002, discloses arrangements of an arrayed waveguide grating (AWG), optical transmitter, and optical communication system including a monitoring function for a main signal. The AWG comprises an input slab waveguide, an output slab waveguide, and a channel waveguide array having waveguides of progressively increasing lengths interconnecting the input and output slab waveguides. A first plurality of input waveguides are coupled to the input slab waveguide, each waveguide being used to launch a separate wavelength signal into the input slab waveguide. In passing through the input slab waveguide, the channel waveguide array, and the output slab waveguide, the launched waveguide signals are multiplexed and recovered at a zero diffraction order output in an output optical fiber. First and second mirrors are located to intercept multiplexed waveguide signals appearing in the first diffraction order beams of the output slab waveguide. Each mirror redirects the intercepted multiplexed waveguide signals back through the output slab waveguide, the channel waveguide array, and the input slab waveguide to demultiplex the waveguide signals for a second time. In the input waveguide slab, the waveguide signals are angularly dispersed to appear at separate locations on the input side of the input slab waveguide that do not coincide with the locations of the first plurality of input waveguides. A first and a second set of a plurality of monitoring waveguides are each disposed at separate locations of the input slab waveguide so that each waveguide of the set receives a separate one of the demultiplexed wavelength signals for monitoring purposes. In a second embodiment, a feedback loop is connected to intercept the first order diffraction beams at the output slab waveguide instead of at the first and second mirrors and to feed the multiplexed signals back through the (AWG). This arrangement is somewhat inefficient because the waveguide signals propagate twice through the AWG and introduces losses to each of the wavelength signals for each pass therethrough.
It is desirable to provide a more efficient grating based wavelength multiplexer/demultiplexer that provides wavelength power taps after only one pass through a grating device, and additionally, if desired, a total power tap. Since a multiplexed signal tap by definition has more power than the individual wavelengths that form the multiplexed signal, higher grating losses are generally acceptable for tapping the total power.